Mosquitoes and biting flies develop worldwide in aquatic environments during temperate months, creating serious public health problems through disease transmission and biting insect annoyance. Control of these public health insects is costly due to the significant labor, equipment and material requirement necessary. To achieve control, all potential breeding sites within the pest insect's flight range must be inspected weekly during summer months, and if found breeding, control materials must be applied. The development of effective, soluble pesticide controlled release formulations for aquatic environments lasting up to 150 days in the early 1980's by Sjogren (U.S. Pat. No. 4,732,762) provided significant labor savings and reduced control costs. This technology has been commercialized using the soluble insect growth regulator S-Methoprene, and has gained widespread acceptance and use.
In the mid 1980's, a microbial pathogen Bacillus thuringiensis israelensis (Bti) was discovered and subsequently commercialized for pest control. In the mid 1990's, a second microbial pathogen of mosquitoes, Bacillus sphericus (Bsph) was also brought to market. These microbial pesticides are highly selective for small immature biting flies (Nematocera), are recognized for their environmental compatibility, and have become control materials of choice for use in mosquito control programs. To be effective these microbial pathogen particles (>1.0 specific gravity) must be applied to the water surface and ingested by aquatic immature biting fly stages, before they settle out of the water column. This is now achieved for short term control by liquid or floating corn cob granules (<½ mesh) applied to flooded field sites that contain mosquito larvae. The duration of control achieved by such liquid or granular treatments vary with the dosage applied, ranging from 24 to 72 hours with Bti, and 7 to 28 days with Bsph.
In 1996, Sjogren (U.S. Pat. No. 5,484,600) described a timed-release microbial granule formulation in the form of a composite particle. The particle is in the form of a dense core, a flotation particle, a pesticide and an adhesive that delivered microbial pesticidal particles to the water surface for mosquito control. In that invention, microbial powders were blended with a buoyant agent and attached to the outside of a sinking granular carrier. A slow dissolving, water soluble protein binder released the buoyant agent and microbial pesticide to the water surface over 10 to 30 days.
Kase (U.S. Pat. No. 4,631,857) describes the delivery of particulate (1.5 to 15 micron) microbial pesticide consisting of a mixture of cork, a larvicidal micro-organism (Bti), gypsum plaster and water sufficient to set the plaster to produce a floating molded unit. When applied into small aquatic field sites, the plaster slowly dissolves releasing the Bti at the water surface. Although the composition is reported to control mosquitoes for up to one month in very small field sites, the invention has a number of disadvantages that are overcome by the present invention. They are: 1) the floating molded unit releases Bti directly into the water as the plaster disintegrates, which limits the lateral dispersal of the sinking particulate pesticide; 2) the floating unit can be beached by winds rendering it ineffective, and anchoring it in place is labor intensive; 3) the unit's relatively large size, about 2 inches diameter, and its light weight, renders it impractical to disperse with mechanical equipment; and 4) its control effectiveness is limited to approximately 30 days.
Particulate pest control compositions used in water perform best when they are evenly distributed in field mosquito breeding sites, and remain in place after application, delivering particulate pathogenic particles at the water surface in their vicinity. Floating compositions readily drift away from where they are applied, and do not achieve the uniform distribution of microbial particles at the water surface in large sites, required for pest insect feeding ingestion. Thus, sinking compositions perform more dependably than floating compositions under diverse field conditions.
The development of an effective sinking sustained release microbial composition which delivers particulate, non-water soluble, microbial pesticides at the water surface over a prolonged period to control aquatic biting flies, is difficult to achieve. For microbial particulate pesticides to be effective they must be uniformly distributed on the water surface of field sites and remain available long enough to be ingested by filter feeding mosquito larvae. Granule and pellet compositions must be small enough in size to permit large numbers to be distributed with mechanical equipment to provide multiple release loci. For such small units to be effective, the composition must be highly efficient to continuously deliver pest controlling quantities of particulate pesticides.
Accordingly, a substantial need exists for a sinking sustained controlled release microbial pesticide composition that remain in place when applied, achieve effective microbial control of pest populations, that provides a continuous release of microbial pesticide to the water surface to maintains an effective mosquito controlling concentration for long periods during the temperate or breeding season.